Freezing to Death

Potted outdoor plants need cold-hardy roots to survive winter

By Dr. Francis Gouin

Did you know that the roots of plants are not as cold-hardy as the stems and branches? What’s more, the roots of different plant species are killed at different temperatures. This is information you need to know when selecting plants for growing in above-ground containers that are to remain outdoors all year long.
Below four feet, soil temperatures seldom drop below 28 degrees. If the ground is covered with mulch or snow, temperatures may be several degrees higher. If the soil is wet at the time it freezes, soil temperatures will also be higher. Dry soils freeze faster and achieve a lower temperature than moist or wet soils.
In above-ground containers, temperatures will equilibrate to the ambient air within hours of a 10-degree change in temperature. The temperature change will occur faster if the rooting medium is dry. A dry rooting medium freezes faster than a wet rooting medium.
If you intend to grow ornamental plants outdoors year-round in above-ground planters, select plants with cold-hardy roots. Choices are many, including Alberta spruce, Amur maple, azaleas, birch, chamaecyparis (or false cypress), mountain laurel, Pfitzer juniper, red cedar, rhododendrons and sumac. The roots of these species can tolerate temperature down to zero and below. However, rooting media should be kept moist during the winter months as well as during the growing season. Avoid planting Atlantic cedar, boxwoods, camellia, Chinese hollies, Colorado spruce, dogwood, Japanese hollies, magnolia and viburnums in above-ground pots. Some of these species have roots that are killed at temperatures as high as 24 degrees. In many years, the roots of these species will be killed before the holidays.
Information on low root-killing temperatures of perennial plants is important when selecting plants for containers and rooftop gardens. On rooftop gardens the problem is not as severe if the garden is installed over a heated building, because heat loss through the roof is generally adequate to prevent rooting media from freezing. However, if the garden is being installed over an unheated parking garage, selecting plants with cold-hardy roots is of utmost importance.